OXYGEN ACTIVITY-DEPENDENT DISSOLUTION OF NICKEL FROM NICKEL-OXIDE INTO PLATINUM

The oxygen activity dependent dissolution of the metallic component of a metal oxide into platinum was experimentally studied in the model s ystem Pt-Ni-O. Thermogravimetric measurements were performed on Pt-Ni1 -DeltaO composite and on Ni1-DeltaO samples to determine mass changes as a function of oxygen activity at 1200, 1300 and 1400 degrees C. (Fo r the sake of simplicity, Ni1-DeltaO is in this article often denoted as NiO, except in cases where Delta is important.) The mass changes of the Pt-NiO composite samples were found to be almost entirely due to the dissolution of Ni from NiO into Pt rather than due to changes in t he deviation from stoichiometry in Ni1-DeltaO, especially at low oxyge n activities. The experimental results suggest that the mole fraction of Ni in Pt-Ni increases with decreasing oxygen activity. From the exp erimental data, the activity of Ni in Pt-Ni alloys and the correspondi ng activity coefficients were determined as a function of composition and compared with data from the literature. By switching oxygen activi ties between two values it was observed that the equilibration of Pt-N iO composites occurs much slower in the reducing direction than in the oxidizing direction. At low oxygen activities the kinetics of the red uction of Pt-NiO composite samples were found to be extremely slow. Th ese slow kinetics are likely due to a very slow incorporation of Ni fr om NiO into Pt-Ni alloys.